Nodule Biomass Research Articles

Kayu Kuku (Pericopsis mooniana Thw.) Seedlings Growth Response to Tailing Media Added with Vermicompost, Rhizobium, and Arbuscular Mycorrhizal Fungi

Kayu kuku (Pericopsis mooniana THW.) has ability to grow excellently on former clay and nickel mining sites but it is unknown how this species can adapt on mercury-contamined soil. The purpose of this study is to analyze the effectivity of vermicompost, rhizobium and arbuscular mycorrhizal fungi (AMF) on improving the seedling growth of P. mooniana on mercury-contamined soil of gold mining tailings media. The study was conducted in a greenhouse using a factorial randomized complete design consisting of 3 factors (vermicompost, rhizobium, and AMF). The application of vermicompost, rhizobium, and AMF could not able to increase the heights, diameters, biomass, and number of root nodules. In contrast, the treatments could increase the amount of chlorophyll, leaf area, primary root length, secondary root number and seedling life percentage up to 92%. Based on root-shoot ratio of P. mooniana seedlings indicated that these seedlings are not suitable to be planted in the field. Symptoms of Hg toxicity seen in seedlings morphology are: leaf and root discoloration, reduction in leaf area, shortening of primary roots, reduction of secondary roots.

Organic or Inorganic Nitrogen and Rhizobia Inoculation Provide Synergistic Growth Response of a Leguminous Forb and Tree.

Our objective was to better understand how organic and inorganic nitrogen (N) forms supplied to a tree, Robinia pseudoacacia, and a perennial forb, Lupinus latifolius, affected plant growth and performance of their symbiotic, N-fixing rhizobia. In one experiment, we tested five sources of N [none; three inorganic forms (ammonium, nitrate, ammonium-nitrate); and an organic form (arginine)] in combination with or without rhizobia inoculation. We measured seedling morphology, allometry, nodule biomass, and N status. A second experiment explored combinations of supplied 15N and inoculation to examine if inorganic or organic N was deleterious to nodule N-fixation. Plant growth was similar among N forms. A positive response of nodule biomass to N was greater in Robinia than Lupinus. For Robinia, inorganic ammonium promoted more nodule biomass than organic arginine. N-fixation was concurrent with robust supply of either inorganic or organic N, and N supply and inoculation significantly interacted to enhance growth of Robinia. For Lupinus, the main effects of inoculation and N supply increased growth but no interaction was observed. Our results indicate that these important restoration species for forest ecosystems respond well to organic or inorganic N forms (or various forms of inorganic N), suggest that the nodulation response may depend on plant species, and show that, in terms of plant growth, N supply and nodulation can be synergistic.

Overexpression of the arginine decarboxylase gene promotes the symbiotic interaction Medicago truncatula-Sinorhizobium meliloti and induces the accumulation of proline and spermine in nodules under salt stress conditions

Legumes have the capacity to fix nitrogen in symbiosis with soil bacteria known as rhizobia by the formation of root nodules. However, nitrogen fixation is highly sensitive to soil salinity with a concomitant reduction of the plant yield and soil fertilization. Polycationic aliphatic amines known as polyamines (PAs) have been shown to be involved in the response to a variety of stresses in plants including soil salinity. Therefore, the generation of transgenic plants overexpressing genes involved in PA biosynthesis have been proposed as a promising tool to improve salt stress tolerance in plants. In this work we tested whether the modulation of PAs in transgenic Medicago truncatula plants was advantageous for the symbiotic interaction with Sinorhizobium meliloti under salt stress conditions, when compared to wild type plants. Consequently, we characterized the symbiotic response to salt stress of the homozygous M. truncatula plant line L-108, constitutively expressing the oat adc gene, coding for the PA biosynthetic enzyme arginine decarboxylase, involved in PAs biosynthesis. In a nodulation kinetic assay, nodule number incremented in L-108 plants under salt stress. In addition, these plants at vegetative stage showed higher nitrogenase and nodule biomass and, under salt stress, accumulated proline (Pro) and spermine (Spm) in nodules, while in wt plants, the accumulation of glutamic acid (Glu), γ-amino butyric acid (GABA) and 1-aminocyclopropane carboxylic acid (ACC) (the ethylene (ET) precursor) were the metabolites involved in the salt stress response. Therefore, overexpression of oat adc gene favours the symbiotic interaction between plants of M. truncatula L-108 and S. meliloti under salt stress and the accumulation of Pro and Spm, seems to be the molecules involved in salt stress tolerance.

Soybean (Glycine max (L) Merrill) Root Growth and Nodulation Responses to Different Soil Moisture Regimes

Climate change due to global warming is contributing to upward shifts in temperatures and reductions in rainfall leading to increased incidences of soil moisture stress. A greenhouse experiment was conducted over two seasons to determine the effect of varying soil moisture regimes on root growth and nodulation of selected soybean (Glycine max (L.) Merrill) cultivars. The experiment was conducted as a Randomized Complete Block Design (RCBD) in a 4 x 6 factorial treatment arrangement with moisture regimes (80, 60, 40, and 20% of field capacity) as first factor and cultivars (Gazelle, Nyala, EAI 3600, DPSB 8, Hill, and DPSB 19) as second factor. Collected data on root diameter, root length, root surface area, root volume, and nodulation were subjected to Analysis of Variance (ANOVA) using Linear Mixed Model in GENSTAT. Significantly different treatment means were separated using Tukey’s test at 0.05 level of significance. Moisture stress significantly reduced soybean root diameter, root length, root surface area, root volume, root biomass, root to shoot ratio, and nodulation of all tested cultivars. The degree of stress however varied with soybean cultivars tested with cultivar EAI 3600 having highest root volume, root biomass, and number of nodules per plant compared to other cultivars. Results suggest that 40% moisture at field capacity could be a threshold moisture stress level for soybean beyond which adaptive soil moisture mitigation practices like supplementary irrigation and use of appropriate agronomic practices be employed to improve soybean yields.

Fungicide application increased copper-bioavailability and impaired nitrogen fixation through reduced root nodule formation on alfalfa

Copper-based fungicides have been used for a long time in viticulture and have accumulated in many vineyard soils. In this study, incrementing Cu(OH)2-based fungicide application from 0.05 to 5 g Cu kg−1 on two agricultural soils (an acidic sandy loam (L, pH 4.95) and an alkaline silt loam (D, pH 7.45)) resulted in 5 times more mobile Cu in the acidic soil. The most sensitive parameters of alfalfa (Medicago sativa) growing in these soils were the root nodule number, decreasing to 34% and 15% of the control at 0.1 g Cu kg−1 in soil L and at 1.5 g Cu kg−1 in soil D, respectively, as well as the nodule biomass, decreasing to 25% and 27% at 0.5 g Cu kg−1 in soil L and at 1.5 g Cu kg−1 in soil D, respectively. However, the enzymatic N2-fixation was not directly affected by Cu in spite of the presence of Cu in the meristem and the zone of effective N2-fixation, as illustrated by chemical imaging. The strongly different responses observed in the two tested soils reflect the higher buffering capacity of the alkaline silt loam and showed that Cu mitigation and remediation strategies should especially target vineyards with acidic, sandy soils.

Vegetation cover affects rhizobia-tree legume symbiosis in the semi-arid region of Brazil

Biological nitrogen fixation (BNF) is the main form of introducing nitrogen into systems with low fertilizer input, which prevail in the semi-arid region of Brazil. BNF was evaluated in leucena and sabia grown in samples of a soil collected under different vegetation cover: native vegetation; capoeira (area abandoned for the restoration); agricultural use; sabia grove; and leucena grove. Nodulation was abundant in soil under all the types of cover. Leucena grown in soil under capoeira exhibited the highest dry biomass of nodules, while the same cover provided the lowest dry biomass of nodules for sabia. Growth and shoot biomass were higher in plants grown in soil under caatinga. BNF was high in leucena and sabia, reaching rates higher than 97% in plants grown in soil under capoeira. The highest amounts of fixed nitrogen (N) were found in leucena plants grown in soil under caatinga. It is the first estimate of the potential to fixed N for sabia and leucena in soils of the semi-arid region, providing an initial estimate of the amounts of N that can be fixed in the field by these tree legumes in the Brazilian semi-arid. Key words: Indigenous rhizobia, isotope, Leucaena leucocephala (Lam.) de Wit, Mimosa caesalpiniifolia Benth, N-15 natural abundance method, tropical dry forest.

Biomass, nodulation and N2 fixing response by subclover and pink serradela to phosphorus fertilization

ABSTRACT In Mediterranean areas, sustainable production of rainfed pastures is associated with plant species which withstand low soil fertility and variable annual drought, phosphorus (P) being the primary limiting nutrient for legume growth and N2 fixation. A study was undertaken under environmentally controlled conditions to compare the response of subclover and pink serradela to P supply using a moderately acid soil, very poor in available P, in terms of biomass, nodulation pattern, N2 fixation and P use efficiency. A P dose in the range of 30–60 kg P ha−1 is recommended as a starter for pastures under the present conditions. A higher P rate reduced the fixing nodule number and biomass, and the N2 fixation rate. Fixed N2 per unit of nodule P by pink serradela as monocrop was about 80% greater than the sole subclover, demonstrating its higher efficiency on the use of nodule P. Thus, pink serradela needs a lower P rate to produce the same amount of biomass per unit area as sole subclover. Long-term studies in field conditions, using these and other new pasture legume species are needed to consolidate the present findings.

Nitrogen fixation and carbon assimilation of the desert legume Tephrosia apollinea under PEG-induced osmotic stress

Tephrosia apollinea (Leguminosae) is a desert legume shrub from UAE deserts and can fix atmospheric nitrogen through Rhizobium symbiosis. Uniform seedlings of T. apollinea were hydroponically grown in beakers (100 ml) covered with sink strainers in a growth chamber and subjected to different levels of polyethylene glycol (PEG) (0%, 5%, 10%, and 15%) treatment. Potential PEG-induced osmotic stress was evaluated through several plasticity traits such as plant biomass, root nodulation and viability of nodules, and stable isotopes of carbon (δ13C) and nitrogen (δ15N). Osmotic stress decreased plant fresh (FW) and dry biomass (DW), with the largest reduction being found at 15% PEG. Osmotic stress significantly decreased δ13C in both roots and leaves, but the reduction was more prominent in the roots. Conversely, osmotic stress significantly increased δ15N in both roots and leaves. We identified four different strains of nitrogen fixing bacteria (Rhizobium leguminosarum, R. radiobacter, R. meliloti and Bradyrhizobium sp.) from root nodules forming a symbiotic association with T. apollinea. The overall results indicate that carbon allocation, root nodule activity, and carbon and nitrogen isotope signatures are useful functional attributes to measure in relation to adaptive survival of T. apollinea in arid deserts. In addition, the combination of ecophysiological and morphological plasticity in T. apollinea suggest it as an interesting biomass source and nurse plant in nutrient-poor soils of desert environments.

Nodule study in Albizia chinensis in relation to nitrogen metabolism, morphology and biomass

Morphology, biomass, nitrate reductase (NR) and nitrogenase activity in Albizia chinensis (Osb.) Merr. nodules were assessed on monthly and seasonal basis for 1 year. Average NR and nitrogenase activity was higher during the rainy season, reaching a maximum in August. Thereafter, activity decreased through autumn and reached a minimum value during winter. Fresh and dry biomass of nodules increased gradually from summer to the rainy season and then started decreasing with the onset of winter as nodules began to senesce. Among four developmental stages of the nodules that correspond to their increasing age, NR and nitrogenase activity remained low in stage 1 nodules and peaked in stage 2. The activity of both enzymes further decreased with increasing age (stage 2 to stage 4). Morphological features such as shape, diameter and color varied considerably among the developmental stages. Stage 1 nodules were cream-colored, oval to heart-shaped with smallest average diameter whereas at stage 2, they became bilobed to tetralobed. On the other hand, stage 3 nodules had the largest average diameter and were multilobed in structure. Stage 4 nodules that correspond to the senescing stage were dark brown to black, multilobed, flattened and hollow due to degeneration of nodular tissue.

Soil-Plant Indices Help Explain Legume Response to Crop Rotation in a Semiarid Environment.

Crop productivity is typically affected by various soil–plant factors systematically as they influence plant photosynthesis, soil fertility, and root systems. However, little is known about how the productivity of legumes is related to crop rotation systems. The objectives of this study were to determine the effect of rotation systems on legume productivity and the relationships among legume productivity and soil–plant factors. Three annual legumes – chickpea (Cicer arietinum L.), pea (Pisum sativum L.), and lentil (Lens culinaris Medikus), were included in various diversified rotation systems and compared with legume monoculture in the 8-year rotation study. Soil N and water conditions, and canopy and root systems were evaluated at the end of 8-year rotation in the semiarid Canadian prairies. Results showed that diversified rotation systems improved leaf greenness by 4%, shoot biomass by 25%, nodule biomass by 44%, and seed yield by 95% for chickpea and pea, but such effects were not found for lentil. Pea monocultures increased root rot severity by threefold compared with diversified rotations, and chickpea monoculture increased shoot rot severity by 23%, root rot severity by 96% and nodule damage by 219%. However, all the legume monocultures improved soil N accumulation by an average 38% compared to diversified systems. Pea and chickpea displayed considerable sensitivity to plant biotic stresses, whereas lentil productivity had a larger dependence on initial soil N content. The 8-year study concludes that the rotational effect on legume productivity varies with legume species, the frequency of a legume appearing in the rotation, and the integration of relevant soil and plant indices.

Rhizobia protect their legume hosts against soil-borne microbial antagonists in a host-genotype-dependent manner

Microbial inhabitants of the rhizosphere can have substantial impacts on the fitness of their associated host plants, in both beneficial and detrimental ways. Soil-borne pathogens can impose severe fitness costs that can be mitigated or eliminated in many cases by co-occurring beneficial bacteria that directly or indirectly temper pathogens’ antagonistic effects. Rhizobial bacteria are best known for their role as nitrogen fixing symbionts in the rhizobia-legume mutualism but there is growing evidence that they can also act as protective agents against microbial pathogens. This study examined the role of rhizobial bacterium Ensifer medicae in protecting the burclover, Medicago polymorpha, against antagonistic soil microbes in complex soil communities. Exposing plants to concentrated slurries of soil inoculum had an adverse effect on all aspects of plant fitness in M. polymorpha. However, inoculating plants with rhizobia increased plant survival in the presence of live soil inoculum from 18% to over 80%. In addition, the soil microbiome changed the symbiotic relationship between plants and rhizobia; plants that were co-inoculated with dilute soil inoculum and a beneficial rhizobium produced more nodules with a higher nodule biomass than plants inoculated with rhizobium alone. Finally, we found that the effects of soil microbes and rhizobia on root biomass, root:shoot ratio, and nodule number differed between host genotypes, indicating there is potential for complex plant-bacterial interactions to respond to selection and potentially contribute to the maintenance of both plant genetic variation and bacterial diversity.

Screening Cowpea Genotypes for High Biological Nitrogen Fixation and Grain Yield under Drought Conditions

Core Ideas Cowpea nodule biomass was more sensitive to water stress compared to shoot biomass. Rainfed conditions resulted in 57% decrease in N2 fixation compared to well‐watered. Inter‐seasonal variability in rainfall accounted for 58% difference in cowpea grain yield. Future increase in drought and heat stress will challenge legume productivity in many regions. In Sub‐Saharan Africa, legumes are earmarked as an integral component in building resilience to climate change and food security in smallholder farming systems. Hence, the need for continuous screening of drought tolerant legume genotypes for sustained productivity. A 2‐yr field study was conducted to identify superior cowpea genotypes with high biological nitrogen fixation (BNF), grain yield, and drought tolerance. Four genotypes, selected from a pool of 91, were grown under four moisture levels ranging from well‐watered (WW) to severe water stress (SS). Grain yield, shoot biomass, nodule mass, and BNF were determined. There was significant effect of water stress on the measured parameters. Nodule mass and BNF were more sensitive to water stress compared to shoot biomass across genotypes. Water stress resulted in 80% reduction in nodule biomass between WW and SS. As a result, cowpea genotypes grown under SS conditions fixed 57% less N2 compared to those under WW. The study also revealed significant genotypic variation in shoot biomass, grain yield, and BNF. Genotype TVu4607 was low in grain yield but performed better for BNF (71 kg N ha−1 under WW and 30 kg N ha−1 under SS). Genotype IT00K‐1263, proved to be our superior genotype with a good balance of biomass, BNF and grain yield. Our findings suggest that future water stress may significantly affect cowpea productivity, mostly its capacity to fix nitrogen. Consequently, adaptation strategies to improve drought resistance in cowpea are needed, particularly improvement of BNF under water stress.

Genetic conflict with a parasitic nematode disrupts the legume-rhizobia mutualism.

Genetic variation for partner quality in mutualisms is an evolutionary paradox. One possible resolution to this puzzle is that there is a tradeoff between partner quality and other fitness‐related traits. Here, we tested whether susceptibility to parasitism is one such tradeoff in the mutualism between legumes and nitrogen‐fixing bacteria (rhizobia). We performed two greenhouse experiments with the legume Medicago truncatula. In the first, we inoculated each plant with the rhizobia Ensifer meliloti and with one of 40 genotypes of the parasitic root‐knot nematode Meloidogyne hapla. In the second experiment, we inoculated all plants with rhizobia and half of the plants with a genetically variable population of nematodes. Using the number of nematode galls as a proxy for infection severity, we found that plant genotypes differed in susceptibility to nematode infection, and nematode genotypes differed in infectivity. Second, we showed that there was a genetic correlation between the number of mutualistic structures formed by rhizobia (nodules) and the number of parasitic structures formed by nematodes (galls). Finally, we found that nematodes disrupt the rhizobia mutualism: nematode‐infected plants formed fewer nodules and had less nodule biomass than uninfected plants. Our results demonstrate that there is genetic conflict between attracting rhizobia and repelling nematodes in Medicago. If genetic conflict with parasitism is a general feature of mutualism, it could account for the maintenance of genetic variation in partner quality and influence the evolutionary dynamics of positive species interactions.

Nodulation and Delayed Nodule Senescence: Strategies of Two Bradyrhizobium Japonicum Isolates with High Capacity to Fix Nitrogen.

The purpose of this work was to study further two Bradyrhizobium japonicum strains with high nitrogen-fixing capacity that were identified within a collection of approximately 200 isolates from the soils of Argentina. Nodulation and nitrogen-fixing capacity and the level of expression of regulatory as well as structural genes of nitrogen fixation and the 1-aminocyclopropane-1-carboxylate (ACC) deaminase gene of the isolates were compared with that of E109-inoculated plants. Both isolates of B. japonicum, 163 and 366, were highly efficient to fix nitrogen compared to commercial strain E109. Isolate 366 developed a higher number and larger biomass of nodules and because of this fixed more nitrogen. Isolate 163 developed the same number and nodule biomass than E109. However, nodules developed by isolate 163 had red interiors for a longer period, had a higher leghemoglobin content, and presented high levels of expression of acdS gene, that codes for an ACC deaminase. In conclusion, naturalized rhizobia of the soils of Argentina hold a diverse population that might be the source of highly active nitrogen-fixing rhizobia, a process that appears to be based on different strategies.

Nitrogen Fertilization of Soybean Affects Root Growth and Nodulation on Two Soil Types in Mississippi

ABSTRACTGreenhouse studies were conducted to evaluate the influence of nitrogen (N) sources [urea + ​N-(n-butyl) thiophosphoric triamide, NBPT (urease inhibitor) and polymer-coated urea (PCU)] and rates on soybean root characteristics, nodule formation, and biomass production on two soil types (silt loam and clay) commonly cropped to soybean in Mississippi. About 15% less belowground biomass was produced in clay soil than in silt loam soil directly corresponding to all other root parameters including root length, root area, root diameter, and nodule number. Pooled across N rates, N additions resulted in 19% and 52% decrease in belowground biomass and number of nodules, respectively, across soils compared to soybean receiving no N. The N rate was the most critical factor as it influenced all root growth parameters. Number of nodules were 24% greater with PCU than urea + NBPT. Nitrogen additions and clay soil negatively impacted soybean root growth, nodulation, and belowground biomass production.Abbreviations: Polymer-coated urea, PCU; N-(n-butyl) thiophosphoric triamide, NBPT

Soil characteristics determine the rhizobia in association with different species of Mimosa in central Brazil

To evaluate the influence of soil type on the symbiosis between Mimosa spp. and rhizobia. A greenhouse experiment was carried out with trap plants using seeds of six species of Mimosa and soils from three different locations in central Brazil: Posse, Brasilia and Cavalcante. Plant dry biomass and number of nodules were measured after four months. Symbiotic bacteria were isolated from nodules and their molecular identification was performed. Three housekeeping genes (16S rRNA, recA and gyrB) plus the nodC and nifH symbiotic genes were used to determine the identity of the symbionts and to reconstruct the phylogenetic relationships among the isolated nitrogen-fixing bacteria. Rhizobia from the Betaproteobacterial genus Paraburkholderia (former Burkholderia) and the Alphaproteobacterial genus Rhizobium were isolated from different species of Mimosa. As in previous studies, the phylogenies of their symbiosis-essential genes, nodC and nifH, were broadly congruent with their core housekeeping genes (16S rRNA, recA and gyrB), which suggests limited or no horizontal gene transfer. Edaphic factors such as pH and fertility influenced the occurrence of these unrelated rhizobial types in the nodules on these Mimosa spp. Mimosa species have the ability to associate with different types of rhizobia (α- and β-proteobacteria), suggesting low specificity between host and bacterium in experimental conditions. Soil factors such as pH, nitrogen and fertility seem to favour the predominance of certain types of rhizobia, thus influencing the establishment of symbiotic relationships.

Wild bradyrhizobia that occur in the Burdekin region of Queensland are as effective as commercial inoculum for mungbean (Vigna radiata (L.)) and black gram (Vigna mungo (L.)) in fixing nitrogen and dry matter production

Anecdotal reports have suggested wild bradyrhizobia (Bradyrhizobium spp.) were as effective as commercial inoculum for mungbean (Vigna radiata (L.)) grown in the Burdekin region of far north Queensland, Australia. To test this hypothesis, we sampled the diversity of bradyrhizobium strains at two field sites in the Burdekin region of north Queensland, and from plants grown in inoculated or uninoculated soil collected from one field site in Millmerran, southern Queensland. We then compared two wild bradyrhizobium with the commercial strain for their ability to fix nitrogen and promote biomass production on plants in a glasshouse experiment. Ten mungbean nodules were collected from each of four plants, from each of two sites in the Burdekin region. One site had been treated with commercial bradyrhizobium inoculum strain CB1015, the other had never been treated with commercial inoculum. Protein analysis of the nodules using matrix assisted desorption ionization time of flight mass spectrometry (MALDI-TOF MS) showed a total of eight different strains of bradyrhizobia, distinctly different to CB1015, were present in the nodules of these plants. Of these eight strains, two strains dominated, one at each of the two collection sites. On the other hand, mungbean plants grown in soils from Millmerran, on the Darling Downs, hosted only CB1015 where inoculated, and did not nodulate when not inoculated. The two dominant wild bradyrhizobia were cultured from nodules from these field collected plants, and used in a controlled glasshouse experiment. In the glasshouse experiment, two mungbean cultivars and one black gram (Vigna mungo (L.)) cultivar were: (i) not inoculated and not supplied with nitrogen, (ii) not inoculated and supplied with nitrogen, (iii) inoculated with the commercial bradyrhizobia strain CB1015, (iv) inoculated with the dominant wild bradyrhizobia strain from Site 1, or (v) inoculated with the dominant wild bradyrhizobia strain from Site 2. Cultivars inoculated with either of the two dominant wild bradyrhizobia strains were similar in biomass and fixed a similar amount of nitrogen, to those inoculated with CB1015. There was no significant difference between the three inoculated treatments for shoot biomass nor nodule biomass per plant. Nodule number did not differ significantly from CB1015 for either of the wild bradyrizobia treatments. For root biomass per plant, the two mungbean cultivars did not differ between the three inoculated treatments, but the black gram had significantly less root biomass compared to the CB1015 treatment for one of the two wild bradyrhizobia treatments. For nitrogen fixed per plant, none of the cultivars showed significant difference between inoculated treatments. Our findings are consistent with the hypothesis that in some parts of Queensland some wild bradyrhozobia may be as effective as commercial inoculum for mungbean and black gram, potentially abrogating the need for artificial inoculation, and perhaps offering new options for the future development of better adapted commercial inoculum.

Nodulation and nitrogen accumulation in pulses vary with species, cultivars, growth stages, and environments

Biological N2-fixation underpins the role of pulse crops in the development of sustainable cropping systems, but it is uncertain how nodulation and N accumulation may differ with pulse species, cultivars, and environments. This 3 yr field study investigated nodulation at the early and late flowering stages and seed and straw N uptake for chickpea (Cicer arietinum L.), dry bean (Phaseolus vulgaris L.), faba bean (Vicia faba L.), field pea (Pisum sativum L.), and lentil (Lens culinaris Medik.). At early flowering, all pulses except dry bean had more nodules in the wetter (2010) than drier year (2009). Faba bean had the most nodules followed by field pea and chickpea, while the nodulation varied with plant growth stages and environments. For both pea and lentil, more nodules were observed at early flowering, but higher nodule biomass was obtained at late flowering. Chickpea had higher nodule biomass at late than early flowering but number of nodules varied with year. Seed N uptake was highest in field pea, whereas straw N uptake was highest in faba bean. Our results suggest a possibility of improving pulse N2-fixation by targeting nodule numbers and nodule biomass, although the outcome of plant N uptake will vary with environments.

Elevated CO2 Increases Nitrogen Fixation at the Reproductive Phase Contributing to Various Yield Responses of Soybean Cultivars.

Nitrogen deficiency limits crop performance under elevated CO2 (eCO2), depending on the ability of plant N uptake. However, the dynamics and redistribution of N2 fixation, and fertilizer and soil N use in legumes under eCO2 have been little studied. Such an investigation is essential to improve the adaptability of legumes to climate change. We took advantage of genotype-specific responses of soybean to increased CO2 to test which N-uptake phenotypes are most strongly related to enhanced yield. Eight soybean cultivars were grown in open-top chambers with either 390 ppm (aCO2) or 550 ppm CO2 (eCO2). The plants were supplied with 100 mg N kg−1 soil as 15N-labeled calcium nitrate, and harvested at the initial seed-filling (R5) and full-mature (R8) stages. Increased yield in response to eCO2 correlated highly (r = 0.95) with an increase in symbiotically fixed N during the R5 to R8 stage. In contrast, eCO2 only led to small increases in the uptake of fertilizer-derived and soil-derived N during R5 to R8, and these increases did not correlate with enhanced yield. Elevated CO2 also decreased the proportion of seed N redistributed from shoot to seeds, and this decrease strongly correlated with increased yield. Moreover, the total N uptake was associated with increases in fixed-N per nodule in response to eCO2, but not with changes in nodule biomass, nodule density, or root length.

Topsoil Stockpiling in Restoration: Impact of Storage Time on Plant Growth and Symbiotic Soil Biota

Addressing plant-soil relationships within restoration science may improve success and reduce costs. Here we assess the question of topsoil storage time: how does stockpile age impact plant biomass and soil microbial activity, particularly root symbionts such as rhizobia and arbuscular mycorrhizal fungi (AMF)? Working in Western Australia and in sandy soils, we grew a legume species <i>Acacia saligna</i> (Fabaceae) in one-, two-, three-, five- and ten-year-old stockpile soils under controlled glasshouse conditions. We assessed whether plant biomass, specific root length, and root diameter decreased with stockpile age. Further, we investigated how stockpile age affected the distribution and the number of effective nodules, nodule biomass and AMF colonization in roots. These above and belowground traits were chosen because they reflect the <i>A. saligna</i> response to growing in soils stockpiled for 1–10 years. We hypothesised that in older stockpiles there would be fewer rhizobial and AMF propagules which would constrain <i>A. saligna</i> growth, whereas in younger stockpiles there would be more rhizobial and AMF propagules, and <i>A. saligna</i> growth would be facilitated via these root symbionts. Using generalized linear mixed models, we found that total plant biomass was the lowest but AMF percent colonization was the highest when <i>A. saligna</i> was grown in ten-year-old soils, suggesting that AMF spores and hyphae are present in old stockpiles. Our results demonstrate that AMF communities may be initially disrupted by soil disturbance and storage, and then begin to re-establish between 5–10 years after stockpiling. However, other soil microbial communities, such as fungal pathogens that were not assessed in this study may have been responsible for decreased <i>A. saligna</i> biomass in older soils. Further research, particularly on other soil microbial communities, is needed to understand restoration success using stockpiled soil older than ten years.